This invention relates to fluid clutches and, more particularly, to magnetorheological fluid clutches having liquid cooling.
Magnetorheological fluid clutches (MRC) have been proposed for various applications requiring a torque responsive control, such as engine cooling fan clutches. More recently, the magnetorheological fluid clutches have been proposed as transmission clutches replacing conventional torque converters as a starting clutch or launch device. A magnetorheological fluid is a suspension of finely powdered magnetizable solids, such as iron or iron alloy, in a suitable fluid medium such as mineral oil, synthetic oil or silicone. A magnetorheological fluid clutch may consist of this type of fluid suspension carried between clutch plates with an associated device providing a desired magnetic flux level across the clutch plates and the fluid. The clutch plates are typically made of a material with high magnetic permeability such as iron. When the magnetic flux is generated across the clutch plates and through the magnetorheological fluid, the suspended particles respond. The response is embodied as an attraction between the clutch plates and the magnetorheological fluid particles. This characteristic phenomenon combined with the internal magnetic attraction between the fluid particles results in torque transmission between the clutch plates. Many of the magnetorheological clutches that have been disclosed in the prior art have been called magnetic particle clutches because they use a suspension of magnetizable particles in a dry powder base. With the development and use of suspensions of magnetizable particles in a fluid medium, studies were conducted into the rheology and features of these suspensions, and consequently, the terminology of magnetorheological fluids as been coined.
The MRC, when used as a launch device, is required to transmit considerably more power than when used as a fan clutch. The MRC, when used as a launch device, can generate a considerable amount of heat that must be expelled from the clutch assembly. As with fan clutches, the MRC launch device relies heavily on air cooling to dissipate the heat generated within it. This limits the size and power capacity of the MRC unless extraordinary methods of cooling air volumes is undertaken. Such increases in air flow result in larger air cooling chambers and large capacity fans or air pumps to provide the air flow volumes necessary to provide the required cooling. As a result of this cooling requirement, prior art application of the MRC as a vehicle launch device have been limited to low-displacement engine (approximately 1.3 L) powertrains. One prior art patent (U.S. Pat. No. 5,823,309 issued Oct. 20, 1998) has proposed the use of transmission hydraulic fluid as a cooling medium. This patent describes a MRC wherein transmission fluid is circulated through a heat exchanger positioned radially inward of a plurality of clutch discs to transfer heat from the clutch to the hydraulic fluid. The majority of the heat rejection passes from the clutch components through the heat exchanger, but the capacity and, therefore, the effectiveness of this heat exchanger are highly limited.
It is an object of the present invention to provide an improved magnetorheological fluid clutch (MRC) having integral liquid cooling. In one aspect of the present invention, a MRC has a coolant inlet port and a coolant outlet port surrounding the output shaft for the MRC. In another aspect of the present invention, a front magnetic core and a rear magnetic core are assembled to contain an encapsulated electromagnetic coil and provided with passages for supplying liquid coolant to and from the coil. In yet another aspect of the present invention, a rear magnetic core and a coolant channel ring have a plurality of axial flow paths that direct liquid coolant toward the encapsulated magnetic coil. In still another aspect of the present invention, a plurality of radial flow paths are formed in the housing and coolant channel ring to direct liquid coolant in a serpentine path from the inlet port, over an outer surface of the outer magnetic core, to the outlet port.
In yet still another aspect of the present invention, flow paths for the liquid cooling medium are formed in the clutch housing, the coolant channel ring, and the rear core. In yet still another aspect of the present invention, a flow divider plate is secured radially inward of the rear core and the channel ring to direct coolant liquid from the inlet port toward radial flow paths in the clutch housing. In a further aspect of the present invention, the flow channel ring is secured to the rear core and has formed therein a plurality of flow paths to direct the liquid coolant from axial flow paths, formed between the housing and the outer magnetic core, to the divider plate and the outlet port. In a yet further aspect of the present invention, the divider plate prevents the intermingling of the inlet coolant flow and the outlet coolant flow. In a still further aspect of the present invention, a portion of the liquid inlet flow in the housing channels is directed to flow past the encapsulated coil and return to join the outlet flow from the flow channel ring at the divider plate.
In operation, the magnetorheological clutch assembly provides torque transfer between an input member and an output member through the contained magnetorheological fluid. When an encapsulated coil is supplied with electrical current from an external source that communicates with the magnetorheological clutch through a conventional interface such as slip rings, an electromagnetic field is established. The magnetic field passes through the input and output members and across the magnetorheological fluid in a cavity which is disposed in working gaps between the input and output members. The magnetic field provides the necessary coupling in an energy transfer process between the input member and the output member. By varying the current level to the encapsulated coil, smooth transmission shifting, gradual torque transfer increases, and substantial lock-up between the input and output members is alternately achieved. Transmission fluid is circulated as a coolant through annular spaces surrounding the inner and outer peripheries of the magnetorheological clutch assembly. Accordingly, an efficient and durable magnetorheological clutch is provided. A fan clutch using a magnetorheological clutch having a structure similar to the assembly disclosed herein, without the cooling of the present invention, is disclosed in U.S. Ser. No. 09/598,327 filed Jun. 20, 2000 and assigned, in part, to the assignee of the present invention.